The present invention relates in general to video processing devices, and in particular to supersampling of digital video output for multiple analog display formats.
Many display devices in use today generate images by coloring each of an array of pixels in accordance with an analog input signal that sequentially specifies the color of each pixel. The analog input signal is generally provided in a format specified by a transmission protocol associated with the device. A number of signal formats are in use today, including “standard definition” television (SDTV) formats such as NTSC (National Television Standards Committee) or PAL (Phase Alternating Line); high definition television (HDTV) formats such as 720p, 1080i or 1080p; and VGA or similar formats for computer monitors. Transmission protocols or signal formats, which are usually defined by some standards body or industry consortium, specify parameters such as the frame rate, the number of lines per frame, the number of pixels per line, the meaning of signal amplitude and/or phase, and similar parameters. For example, the NTSC protocol specifies a data rate 525 lines per frame and about 30 frames, interlaced, per second. It also specifies a relationship between (analog) signal amplitude and pixel intensity and, in the case of color images, between signal phase and pixel hue. Such specifications establish a bandpass and other characteristics of the analog signals that a video data source should provide.
In general, different protocols specify different signal formats, and display devices are usually designed for a single format. Thus, makers of video processing devices and other video data sources are confronted with the challenge of providing video signals for a number of different (and sometimes still evolving) formats.
One solution is to provide a different video processing device for each different format. This, however, makes it harder for the end user to upgrade one component of a system, e.g., replacing an SDTV display device with an HDTV, because any incompatible video processing devices (video game consoles, DVD players, etc.) would also have to be replaced. It also requires the manufacturer to design and build a number of different devices with different internal architectures, adding overhead.
Another, more common, solution is to provide a video processing device that has different output processing paths for some number of different standards. For example, FIG. 1 shows block diagrams for conventional output paths of a video card capable of providing data to an SDTV display and a computer monitor. FIG. 1A shows a separate processing path for each type of device. The TV path 102 includes a pixel pipeline 104 that supplies digital pixel data (e.g., RGB color components); an encoder 106 that converts the pixel data to samples of an analog signal for the TV (e.g., in NTSC format); a digital to analog converter (DAC) 108; and a reconstruction filter 110, which generally includes a low-pass filter combined with one or more correction elements (e.g., a sin(x)/x correction) that is designed to reduce artifacts in the analog signal resulting from DAC 108 and the low-pass filter section of recon filter 110. The monitor path 120 includes a pixel pipeline 122; an encoder 124; a DAC 126; and an electromagnetic interference (EMI) filter 128, which is simply a low-pass filter with a frequency cut off above about 200 MHz. Such filters are commonly provided to limit the high frequency radiation emitted by electronic devices (e.g., in compliance with Federal Communications Commission (FCC) regulations in the United States). The arrangement of FIG. 1A entails duplication of numerous components, including the pixel pipeline, the encoder, and the DAC. This duplication wastes card or chip area and complicates the designer's task.
FIG. 1B shows an alternative conventional design in which some elements common to both processing paths can be combined. Thus, there is just one pixel pipeline 132, one encoder 134 (which may be configurable for different analog formats) and one DAC 136. Analog switch 138 is provided to direct the signal to either a reconstruction filter 140 of a TV path or an EMI filter 142 of a monitor path. While this arrangement reduces duplication of components, analog switch 138 introduces additional complexity to the design and can result in loss of signal integrity. In addition, to modify the card to support a third output format, it would generally be necessary to provide a three-way analog switch and additional filters appropriate to the third format. This would require at least some redesign and limits the number of display devices that a single video processing card can be adapted to drive.
It would therefore be desirable to provide an improved video data source that supports multiple standards and can easily be reconfigured for different standards.